This section provides background information related to the present disclosure which is not necessarily prior art.
In a typical data center, the UPS systems used to provide power to data centers need to manage short circuit conditions which may occur from power distribution or load failures. These UPS systems typically have a capacity of 40 kVA or higher. Short circuit currents may be many times higher than the rated load of an UPS system. If they are not interrupted, they can cause the disruption of the equipment (such as loss of basic insulation, arcing from live parts, and the like). Consequently, overcurrent protection devices such as circuit breakers or fuses are provided upstream of the UPS system to limit the short circuit in peak and duration and downstream of the UPS system to protect loads coupled to the UPS. If a load downstream of the UPS system experiences a short circuit, the UPS system needs to “clear” the short circuit quickly enough so that the other loads coupled to the UPS system are not affected. In general, the period of time that the UPS has to clear the short circuit is the backup time of a typical load. Loads in data centers, such as servers, coupled to a UPS system usually have a short period of time during which they will continue to operate in the event of a power interruption, which is commonly known as the load backup time. The UPS system clears a short circuited load by feeding sufficient current to the short circuited load to trip the circuit protection device protecting the load, that is, to trip a circuit breaker if the overcurrent protection device is a circuit breaker or blow a fuse if the overcurrent protection device is a fuse. This disconnects the short circuited load from the UPS system taking the load offline. The UPS system can then continue to provide power to the other loads coupled to the UPS system.
The minimum load backup time is usually 10 ms. This means that the power outage (or interruption) due to clearing a short circuit of a load needs to be less than 10 ms to avoid affecting the other loads coupled to the UPS system. This means that the overcurrent protection devices protecting the load must open within this time period. To achieve this usually means that a fuse (or fuses in the case of multi-phase power) is used as the overcurrent protection device for the load.
When the UPS system feeds the load through the static transfer switch (low impedance path), overcurrent protection at the load can be easily implemented with slow blow fuses rated at a nominal current which can also be very close to the nominal current of the UPS system. For those situations where the short circuit clearance is accomplished by feeding the load by the inverter of the UPS system and a slow blow fuse is used to protect the load, there is a risk that the time that it will take to blow the fuse will exceed the minimum back-up time. To ameliorate this risk, a fuse rated less than the nominal output current of the UPS system can be used to protect the load. Other solutions are to use oversize inverters in the UPS system or use a fast blow fuse to protect the load instead of a slow blow fuse, both of which tend not to be cost effective.
FIG. 1 is a simplified schematic of a typical prior art UPS system 100. The basic elements of UPS system 100 are rectifier 102, inverter 104, a DC power source such as battery 106, a controller 108, and a static transfer switch 110. Battery 106 may be coupled through a boost circuit 107 to an input 105 of inverter 104, which is also coupled to an output 103 of rectifier 102. An input 114 of rectifier 102 is coupled through disconnect switch 116 to a primary power source 115 of power, typically an AC feed from a utility. An input 118 of static transfer switch 110 is coupled through disconnect switch 120 to a secondary power source 122 of power, typically an AC feed from a utility, and an output 124 of static transfer switch 110 is coupled to an output 126 of inverter 104. Output 126 of inverter 104 is coupled through a disconnect switch 128 to output 112 of UPS system 100. Output 112 of UPS system 100 is coupled through a manual bypass switch 130 to secondary power source 122. It should be understood that primary power source 115 and secondary power source 122 can be different power sources or the same power source, such as the same utility feed coupled to both disconnect switches 116, 120. Static transfer switch 110 is used to switch load 134 connected to an output 112 of UPS system 100 to secondary power source 122. A fuse 138 is used to protect load 134 and is coupled in series between load 134 and the output 112 of UPS system 100. In this regard, when static transfer switch 110 is closed, the load is connected to secondary power source 122 and when static transfer switch is open, the load is disconnected from secondary power source 122 (unless the manual bypass switch 130 has been closed).
Controller 108 is configured to control UPS system 100 including controlling inverter 104 by varying the duty cycle of the switching devices in inverter 104 so that inverter 104 provides a desired output voltage. Controller 108 also controls static transfer switch 110 to cause it to switch between closed and open. Controller 108 can be, be part of, or include: an Application Specific Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); and/or a processor such as a Digital Signal Processor (DSP), microcontroller, or the like. It should be understood that controller 108 may include one or more than one of the foregoing, such as digital controller based on DSPs that control each of the functional blocks of UPS system 100 by generating the proper switching signals to switch the power semiconductors such as IGBTs and thyristors.
Rectifier 102 may be a three phase rectifier having three full rectification legs (and illustratively uses power switching devices such as IGBTs), one for each phase, and inverter 104 may be a three phase inverter having three inverter legs, one for each phase. Inverter 104 also illustratively uses power switching devices such as IGBTs. Rectifier 102 and inverter 104 are configured in a double conversion path with UPS system 100 thus being a double conversion UPS.
Static transfer switch 110 is typically implemented with power semiconductor switching devices. One type of power semiconductor switching device used in implementing static transfer switches is the thyristor since it is a very robust device, is relatively inexpensive, and has low losses. Typically, a static transfer switch implemented with thyristors has a pair of reverse connected thyristors 132 for each phase. That is, if UPS system 100 is a three phase system, static transfer switch 110 would have three pairs of reverse connected thyristors 132, one for each phase. It should be understood that each thyristor 132 may include a plurality of parallel connected thyristors 132 to provide the requisite power handling capability.